I know this is late, but my mid 2009 MBP has been running Sierra for a few days now and apart from some thermal issues at the start when it was setting everything up. Now it runs pretty smoothly and performance is comparable to El Capitan. I used the dosdude method and patcher, just make sure you back up the HD before you go ahead
My spec:
Mid 2009 MacBook Pro 13"
Intel Core 2 Duo @ 2.26GHz
4GB DDR3 @1,066MHz
350GB HDD